This invention relates to an image reading apparatus which is suitable for a digital copier in which a laser beam is utilized.
An image processing apparatus, for example, a digital copier to which a laser beam is applied, is composed of the scanning part 300, the image processing part 400, and the printing part 100 as illustrated in FIG. 2.
When the scanning part 300 is operated and the document 200 is optically scanned, the image information of the document 200 is converted into an optical image. This optical image is supplied to the image processing part 400 to be converted into an image signal and the image signal is processed, wherein image processing comprises magnifying and reducing, halftoning, outline processing and so forth. When a color image is processed, the image processing includes color ghosting correction processing.
In the printing part 100, an image is recorded according to the digital image signal (the image data) which is composed of a predetermined number of bits.
FIG. 3 is a schematic illustration which shows an example of the printing part 100. In this example, an electrophotographic printer to which a photoreceptor drum is applied, is used with laser beams as the light source to form an electrostatic latent image on the photoreceptor drum.
In FIG. 3, the image data DATA outputted from the image processing part 400 is supplied to the modulation part 110 in which a signal is formed according to the image data DATA.
The signal outputted from the modulation circuit 110 is supplied to the semiconductor laser 931 and image forming is conducted. The laser driving circuit 932 is controlled by the control signal sent from the timing circuit 933 so that the laser driving circuit 932 can be driven only when the image is formed in the horizontally and vertically effective range.
The signal to indicate the amount of laser beams is fed back to the laser driving circuit 932 from the semiconductor laser 931 and the semiconductor laser 931 is controlled by the laser driving circuit 932 so that the amount of laser beams emitted by the semiconductor laser 931 can become constant.
The laser beam emitted from the semiconductor laser 931 is supplied to the polygon mirror 935 and deflected. The scanning start point of the laser beam which was deflected by the polygon mirror 935, is detected by the index sensor 936. The detected signal is converted into a voltage signal by the I/V amplifier 937 so that the index signal SI can be formed, wherein the index signal SI is not illustrated in the drawing. The index signal SI is supplied to the control means which controls the timing of optical scanning in the scanning part 300.
The numeral 934 is a motor driving circuit which rotates the polygon mirror 935. The control signal is supplied to the motor driving circuit 934 from the timing circuit 933.
FIG. 4 illustrates an example of a laser beam scanner in which image-formation is conducted. The laser beam emitted from the semiconductor laser 931 falls on the above-described polygon mirror 935 through the mirrors 942 and 943. The laser beam is deflected by the polygon mirror 935. The deflected laser beam is irradiated on the surface of the photoreceptor drum 130 through the f-.theta. lens 944 which is used in order to change the laser beam to the beam with a predetermined diameter.
The numerals 945 and 946 are cylindrical lenses which are used to correct the inclined angle.
The laser beam scans the surface of the photoreceptor drum 130 in the prescribed direction "a" at a constant speed by the polygon mirror 935. The surface of the photoreceptor drum 130 is exposed to the laser beam in accordance with the image data so that an electrostatic latent image can be formed.
The toner which is charged to the reversed polarity adheres to the latent image on the photoreceptor drum and the latent image is developed, which is not illustrated in the drawing. Then, a recording paper is put on the toner image. Electrical charge of the polarity which is reverse to that of the toner, is impressed on the reverse side of the recording paper by a corona charger so that the toner image can be transferred onto the recording paper. Furthermore, heat or pressure is given onto the transferred toner image so that the transferred toner image can be fixed onto the recording paper. FIG. 5 is a schematic illustration which shows an example of the scanning part 300.
In FIG. 5, the numeral 1 is a CCD line formed sensor. The output signal of the line formed sensor 1 is supplied to the input terminal of the differential amplifier 5 through the buffer 2, the sample holding circuit 3, and the clamp circuit 4, wherein they are connected in series.
The output signal of the differential amplifier 5 is supplied to the flash type A/D converter 6 of 8-bits, for instance, and converted into the digital signal. The standard voltage Vr, for example -2V, is supplied to the A/D converter 6. The level of the analog signal corresponding to the digital signal FFH is set by the standard voltage Vr. To be more concrete, the digital signal of OOH to FFH is outputted by the A/D converter 6 corresponding to the voltage 0V to -2V. In other words, the full conversion scale of the A/D converter 6 is 2V. The output signal of the A/D converter 6 is supplied to the black level correction signal producing circuit 7. The black level correction signal producing circuit 7 is provided with the memory in which the black level correction signal is written. The controller 8 controls the writing into the memory and the reading out from the memory.
In this case, when the recording key (the copy key) 9 connected with the controller 8 is pressed, firstly a test mode is carried out so that the light source (not illustrated in the drawing) which irradiates the document 200 (Refer to FIG. 2) is turned off. At this moment, the output signal of the line sensor 1 is only composed of the dark current component. This dark current component is converted into the digital signal by the A/D converter 6 and supplied to the black level correction signal producing circuit 7. In the black level correction signal producing circuit 7, averaging is conducted and one line of black level correction signal is produced. This black level correction signal is written in the memory to be stored. In this case, the level of the signal supplied to the input terminal of the differential amplifier 5 is 0.
After a line of black level correction signal is produced by the black level correction signal producing circuit 7 and written in the memory, a normal reading mode is carried out so that a line of black level correction signal is repeatedly read out at each line with the signal being converted into an analog signal by the D/A converter 10. Then, the signal is supplied to the input terminal of an differential amplifier 5 through the buffer 11.
In this case, the light source is turned on and the output signal of the CCD line sensor 1 becomes the image signal which includes the dark current component. This image signal is supplied to the input terminal of the differential amplifier 5. Accordingly, the image signal from which the dark current component is eliminated, is outputted from the differential amplifier 5. This image signal is outputted as the image data DATA of 8-bits through the A/D converter 6.
According to the example illustrated in FIG. 5, the image data DATA from which the dark current component was eliminated can be obtained from the A/D converter 6.
However, in the example illustrated in FIG. 5, the full conversion scale of the A/D converter is 2V, which is the same when the dark current component is sampled in the A/D converter 6 and the black level correction signal is produced in the black level correction signal producing circuit 7. However, taking into consideration that the dark current component is generally some 10 mV, the accuracy of the above-described sampling is not good and it has the disadvantage that the wave-form of the image signal which is outputted from the differential amplifier 5 after black level correction, is deteriorated.
For example, assume that a temperature difference has occurred in the longitudinal direction of the line formed sensor 1, and that the dark current component reciprocating with each pixel of the line formed sensor 1 has occurred as illustrated in FIG. 6A, wherein the dark current component is inclined in the logitudinal direction of the sensor 1. When the dark current component described above is converted by D/A conversion after having been converted by A/D conversion with the full conversion scale 2 V, the result is as shown in FIG. 6B. Accordingly, the corrected image signal which is outputted from the differential amplifier 5 is shown in FIG. 6C. Incidentally, when the full conversion scale is 2 V, the quantization level is 2/256V=7.8mV.